Le mercure, polluant toxique pour les êtres vivants est présent dans tous les compartiments de l'environnement. Son cycle biogéochimique est encore mal connu, et des lacunes existent, notamment dans les sites de hautes altitudes comme les Alpes françaises. Au cours de ces travaux, nous avons examiné la dynamique du mercure dans les écosystèmes lacustres alpins, par l'étude du transfert de ce polluant entre les différents réservoirs que sont la neige (bassin versant), l'eau des lacs, et les poissons. Par des échantillonnages réguliers (janvier à juin 2009) de la neige de surface du lac Bramant, nous montrons que le manteau neigeux est le siège de processus de dépôts atmosphériques de mercure et de méthylmercure provenant des sources anthropiques de la région de Grenoble. Nous montrons également qu'une proportion de ces espèces rejoint le lac notamment lors de la fonte du manteau neigeux. Cependant, elles s'avèrent rapidement évacuées du lac en raison d'un fort et rapide renouvellement de ces eaux. Par ailleurs, la caractéristique spécifique (peu de végétation, grande minéralité) de ces bassins versants d'altitude explique également les faibles quantités de mercure présentes dans ces lacs. La faible contamination en mercure retrouvée dans les poissons de ces lacs est certainement due notamment à un faible temps de séjour du mercure dans l'eau. Enfin, grâce à un outil de prédiction et d'aide à la décision (WARMF), nous suggérons que les paramètres essentiels gouvernant la méthylation du mercure et la contamination des poissons dans nos lacs ne sont pas les dépôts atmosphériques mais plutôt les caractéristiques physico-chimiques du lac et de son bassin versant.

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal.

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal

Increases in global temperatures resulting from climate change have raised concern over potential responses of Arctic charr, <i>Salvelinus alpinus</i>, a cold-adapted freshwater/anadromous fish species in the salmonid family. While various aspects of adult Arctic charr ecology are well established, juvenile, and in particular, young-of-the-year (YOY) ecology is less well understood. The study of early life stages is important because of implications for subsequent population dynamics as well as their particular sensitivity to climate change. In this thesis, I aimed to broaden the scope of knowledge on juvenile Arctic charr from Canadian populations with a particular emphasis on YOY, and thermal habitat use through four studies. An intensive study of juvenile Arctic charr from Lake Hazen, Nunavut, demonstrated a preference during the summer for stream environments, particularly those fed by warm upstream ponds. Charr occupying both stream and nearshore lake habitats were found to feed similarly, with chironomids occurring most frequently in diets. Some older stream-dwelling charr preyed on smaller, younger Arctic charr. Preferred stream occupancy is likely mediated by physical barriers created mainly by water velocity, and by distance from the lake, lake-ice dynamics, low water depth, and turbidity. Water velocities and possibly intercohort competition resulted in stream habitat segregation by size, with YOY mainly found in low velocity pools and back eddies adjacent to stream banks, but not in water velocities greater than 0.1m/s. Greatest charr densities in streams were found in small, shallow, slow-flowing side channels, which are highly susceptible to drought. A discriminant function analysis model based on carbon and nitrogen stable isotope values was used to identify offspring of piscivorous large-form and non-piscivorous small-form Arctic charr morphotypes from Lake Hazen, Nunavut. The adult morphotypes were estimated to contribute approximately equally to the YOY population, however, the morphotype offspring were differentially distributed among sampled nursery sites. Unequal distribution corresponds with prerequisites suggested for the evolution of trophic specialists within a single population that experiences assortative mating based on trophic specialization. An assessment of growth rates and otolith-inferred metabolic rates obtained from YOY from 23 populations over a 27˚ latitudinal gradient demonstrated latitudinal variability consistent with countergradient variation where northern populations demonstrated faster growth and higher metabolic rates compared to southern populations. Countergradient variation (CGV) is defined as genetic variation that counteracts the negative influences of the physical environment, minimizing phenotypic variability along a gradient. Otolith-inferred metabolic rates from free-living animals reflect the average daily energy expenditure of the organism, which incorporate the energetic costs of standard metabolic rate (SMR) and other processes such as feeding, locomotion, thermoregulation, reproduction and growth. As such, variations in otolith-inferred metabolic rates may reflect a combined increase in feeding, activity and SMRs in northern populations. Nevertheless, the phenotypic variation in physiological traits observed here demonstrates the significant adaptability of Arctic charr to different thermal regimes with different growing season lengths. Otolith-inferred temperatures and fork lengths at capture from YOY from two proximal fluvial and lacustrine sites in Labrador were used to compare growth and thermal habitat use between habitat types. Otolith-inferred temperatures were not significantly correlated with air temperatures, suggestive of behavioural thermoregulation by YOY at both sites. The majority of YOY from Kogluktokoluk Brook (fluvial) were found using temperatures consistent with laboratory determined preferred temperatures for juvenile Arctic charr, whereas most Tom’s Pond (lacustrine) YOY were found using temperatures ranging between preferred temperatures and optimal temperatures for growth. Otolith-inferred temperatures were only correlated to fork lengths in Tom’s Pond YOY. The lack of correlation in Kogluktokoluk Brook YOY may reflect resource partitioning occurring as a result of territoriality known to occur among stream salmonids. The limited range of temperatures used by fluvial YOY in this study, particularly the lack of cooler temperatures, suggests that fluvial YOY may face barriers to accessing thermal refugia, and as a result may be particularly vulnerable to climate change. Examining the ecology of juvenile Arctic charr from Canadian populations over a number of spatial scales (i.e. latitudinal, regional and local) highlighted the considerable phenotypic plasticity demonstrated by the species. While physiological plasticity observed over the latitudinal gradient reflected the ability for juvenile Arctic charr to utilize different thermal regimes, the regional comparison between habitat types demonstrated that the ability for juvenile Arctic charr to respond to climate change is likely to vary between habitat types. Further, on a local scale, behavioural plasticity was observed, but was found to be influenced by several regulatory factors. The study of the ecology of juvenile Arctic charr in this thesis has highlighted various factors affecting juvenile Arctic charr in Canada: temperature, water velocity, cover, maternal influences, habitat type and ration. The relative contributions of these factors as well as others which were not directly testable in this thesis (e.g. variability in standard metabolic rate, the role of genetic adaptation) are likely to vary with latitude, populations and habitat types. Deciphering the relative roles of these factors will allow better predictions of responses to climate change.

La presente Tesis Doctoral tiene como objetivo profundizar en los conocimientos sobre ritmos biológicos y comportamiento alimentario de tres especies de peces teleósteos de interés en acuicultura: tenca (Tinca tinca), trucha alpina (Salvelinus alpinus) y lubina (Dicentrarchus labrax).Con este fin se ha investigado la influencia de factores bióticos y abióticos en la sincronización de los ritmos de actividad locomotora y alimentaria, así como el carácter endógeno y/o exógeno de estos ritmos. A su vez, se ha profundizado en el comportamiento individual de truchas alpinas mantenidas en grupo, mediante una nueva metodología que permite estudiar los ritmos de demanda voluntaria de alimento y la autoselección dietaria de los individuos. Asimismo, se ha analizado la influencia de los niveles endógenos de melatonina, modificados mediante la administración de melatonina exógena y de su aminoácido precursor (triptófano) en la dieta, sobre la concentración de cortisol y el ritmo de actividad locomotora en lubina. / This Doctoral Thesis deeps into the knowledge about biological rhythms and feeding behaviour in three teleostean fish species of interest in aquaculture: tench (Tinca tinca), Arctic charr (Salvelinus alpinus) and European sea bass (Dicentrarchus labrax). With this aim, the influence of biotic and abiotic factors has been researched in the field of synchronization of locomotor and feeding rhythms, as far as the endogenous or exogenous character of these rhythms. Moreover, individual feeding behaviour of Arctic charr kept in groups has been studied trying a new methodology which allows the monitoring of feeding demands and dietary self-selection of individuals. Furthermore, influence of endogenous melatonin modified through exogenous melatonin and its precursor amino acid (tryptophan) administration in the diet, in the cortisol levels and locomotor activity rhythms in sea bass were analysed.